Flow-through conductivity sensor

ABSTRACT

An improved flow-through conductivity sensor is provided. The sensor includes a current return path that has a current return conductor. At least one toroid of the sensor is removed from the flow path and configured to interact with the current return conductor to provide an indication of conductivity. Additional aspects of the invention include disposing a pair of toroids about the current return conductor; disposing a toroid about the current return conductor and configuring the toroid as a transformer.

FIELD OF THE INVENTION

The present invention is related to devices and systems that measureconductivity. More particularly, the present invention relates toflow-through type conductivity sensors.

BACKGROUND OF THE INVENTION

Conductivity measurement sensors are well known in the art and are usedto measure the conductivity of a fluid, such as a liquid or a dispersionof solids suspended in a liquid. Conductivity sensors are often used toinvestigate the properties of electrolytes in solution, such as thedegree of disassociation, the formation of chemical complexes, andhydrolysis. The conductivity of a fluid may also be used to measure awide variety of other parameters, such as the amount of contaminants indrinking water and a measure of chemical concentrations in industrialprocess streams. Applications such as these involve the determination ofconductivities in many different physical environments.

Toroidal conductivity sensors generally include two toroidal coils. Thefirst coil is electrically excited by an alternating current source togenerate a changing magnetic field. The changing magnetic field inducesan electrical current in the liquid. In electrolytic solutions, themechanism of electrical current transfer is dependent on ions. Themagnitude of the induced current is indicative of the conductivity ofthe liquid. The second coil detects the magnitude of the inducedcurrent. Typically, toroidal conductivity sensors are best suited foruse in processes where conventional conductivity sensors, such as thosewith electrodes exposed to the measured solution, would corrode orbecome foul.

One example of a toroidal conductivity sensor is sold under the tradedesignation Model 242 available from the Rosemount Analytical,Incorporated Division of Emerson Process, which division is located inIrvine, Calif. The Model 242 sensor is designed to be installed easilyinto process piping between mounting flanges. As a flow-throughconductivity sensor, the Model 242 is not sensitive to flow rate ordirection and it does not obstruct the flow of process fluid. Typically,a flow-through conductivity sensor, such as the Model 242 iselectrically coupled to a compatible instrument such as instrumentmodels 54eC, 1055, 3081T, 4081T and 5081T, all of which are availablefrom the Rosemount Analytical, Incorporated Division of Emerson Process.

Toroidal conductivity sensors of the prior art have generally performedwell. One limitation of such sensors, however, has been that whensensors are offered for different pipe sizes, different sized toroidsmust be manufactured to accommodate the various pipe sizes. Once acompany offers three or four pipe sizes as well as toroids havingdifferent winding counts for each pipe size, the sheer number ofdifferent toroids that must be manufactured grows quickly. This tends todrive up manufacturing costs since the individual lots of toroids arerelatively smaller. Providing a toroidal-type conductivity sensor thatcould use standardized toroids would allow manufacturing of such toroidsto be done on a much larger scale and thus the component cost of thetoroid reduced thereby also reducing costs of the final unit.

SUMMARY OF THE INVENTION

An improved flow-through conductivity sensor is provided. The sensorincludes a current return path that has a current return conductor. Atleast one toroid of the sensor is removed from the flow path andconfigured to interact with the current return conductor to provide anindication of conductivity. Additional aspects of the invention includedisposing a pair of toroids about the current return conductor;disposing a toroid about the current return conductor and configuringthe toroid as a transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are side elevation and perspective views, respectively, ofa flow-through conductivity sensor in accordance with the prior art.

FIG. 3 illustrates, diagrammatically, the operation of the drive toroid,the receive toroid, and the current return path.

FIG. 4 is a diagrammatic view of a flow-through conductivity sensor inaccordance with an embodiment of the present invention.

FIG. 5 is a diagrammatic view of a flow-through conductivity sensor inaccordance with another embodiment of the present invention.

FIG. 6 is a diagrammatic view of a flow-through toroidal conductivitysensor in accordance with another embodiment of the present invention.

FIG. 7 is a diagrammatic view of a pair of electrodes usable withembodiments of the present invention when process piping has aconductive inner surface.

FIG. 8 is a diagrammatic view of a pair of electrodes formed withcommercially available flange pipe in accordance with embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 are side elevation and perspective views, respectively, ofa flow-through conductivity sensor in accordance with the prior art.Sensor 10 is adapted to mount to process piping 12 in order to measureconductivity of process fluid flowing therethrough. Sensor 10 includesbody 14 which is generally a two-part construction from halves 14 a and14 b (illustrated in FIG. 2). A number of holes 16 within each of thehalves 14 a, 14 b are sized to receive mounting bolts 18 and allow thebody to be fixed to pipe flanges using mounting bolts 18 and suitablefasteners 20. Junction box 22 is generally fixed to the top of sensor 10and includes any suitable wiring terminations and connections, as may beappropriate. Since it is generally known that conductivity can vary as afunction of temperature, sensor 10 generally includes a temperaturesensitive device 24 that is adapted to provide an indication of processfluid temperature flowing through sensor 10 such that the conductivitymeasurement can be compensated for temperature.

Referring specifically to FIG. 2, toroid housing 26 is disposed betweeneach of halves 14 a and 14 b and contains both the drive and receivetoroid. As illustrated, toroid housing 26 is sized such that the processfluid flows through the toroid housing 26 and thus through both thedrive toroid and the receive toroid. FIG. 2 also shows a pair ofcontacting rings 28 which are generally adapted to make electricalcontact with the fluid flowing therethrough. These contacting rings 28are also electrically coupled to body 14 which itself is generallyconductive. Accordingly, the combination of a pair of contacting rings28 and the metal housing of the sensor itself provide a current returnpath for the current excited in the fluid as a result of operation ofthe drive toroid.

FIG. 3 illustrates, diagrammatically, the operation of the drive toroid,the receive toroid, and the current return path formed by virtue of thecontacting rings 28 and the conductive housing 14 of sensor 10. Drivetoroid 30 is disposed about process fluid flow path 32. Receive toroid34 is also disposed about flow path 32. The application of analternating current to conductors 36, wound about drive toroid 30,generates a changing magnetic field that induces a current within theconductive process fluid within flow path 32. This current is detectedby receive toroid 34 at conductors 38 which are wound about receivetoroid 34 and coupled to suitable detection circuitry (not shown). Inorder to create a closed circuit, current return path 40 is provided. Inthe prior art, current return path 40 is generally formed by conductiverings being electrically coupled to a conductive sensor housing. Asillustrated in FIG. 3, prior art toroidal flow-through sensors provideboth toroids (drive and receive) about the flow path. As set forthabove, the flow path may be a pipe of varying sizes depending on theapplication. Accordingly, design and manufacture of the flow-throughsensor is somewhat customized depending on the size of the flow pathsince toroids must be designed that can fit around the flow path whilestill effectively coupling the magnetic fields to the flow path.

Many embodiments of the present invention will be described with respectto process piping that has a non-conductive inner surface. However,embodiments of the present invention are equally practicable withprocess piping that has a conductive inner surface. Suitable electrodesfor these applications are described in greater detail with respect toFIGS. 7 and 8.

FIG. 4 is a diagrammatic view of a flow-through conductivity sensor inaccordance with an embodiment of the present invention. In accordancewith one broad aspect, sensor 100 is similar to sensors of the prior artwith two important exceptions. First, at least one of toroids 102, 104is removed from the flow path. Specifically, toroid 104 has beendisposed about current return conductor 106. Current return conductor106 also differs from current return paths of the prior art in that itis actually a conductor, such as a wire. One example of current returnpath 106 might include a pair of contact rings, such as contact rings28, but which rings 28 are then electrically isolated from theconductive sensor body. An electrical conductor, such as a wire, is thenelectrically coupled to each of the contact rings 28 in order to forceall return current through the conductor. This configuration then allowsa toroid 104 to be disposed about the conductor to interact with thecurrent. Embodiments of the present invention include toroid 102 being adrive toroid and toroid 104 being a receive toroid; or toroid 104 beinga drive toroid and toroid 102 being a receive toroid. This configurationis an improvement over the prior art in that at least one toroid can beformed with a standardized size (particularly toroid 104 illustrated inFIG. 4). Those skilled in the art will appreciate that the constructionof current return path 106 can be done in any suitable manner in which apair of electrical conductors are brought into contact with the processfluid and where such conductors are coupled to one another through anelectrical conductor of suitable size to have a toroid disposedthereabout. For example, when the process piping itself isnon-conductive, the connection of the electrodes to the process pipe canbe realized by using built-in pipe thread or flange of the electrodes.Thus, the electrodes can take any form as long as conductive electrodematerial is brought into contact with the process fluid.

FIG. 5 is a diagrammatic view of a flow-through toroidal conductivitysensor in accordance with another embodiment of the present invention.The embodiment illustrated in FIG. 5 extends the benefits of the presentinvention by disposing both toroids about the current return path. Thus,one of toroids 104, 110 is a drive toroid while the other is a receivetoroid. In this embodiment, both toroids 104 and 110 can be constructedfrom a standardized design that does not vary as the pipe diametervaries. Accordingly, toroids 104 and 110 can be more cost effectivelymass-produced while still providing benefits of non-contact conductivitymeasurement for sensor 108.

FIG. 6 is a diagrammatic view of a flow-through toroidal conductivitysensor in accordance with another embodiment of the present invention.Sensor 120 includes a single toroid configured to act as a transformer.Accordingly, toroid 122 includes a first winding 124; and a secondwinding 126 electrically coupled in series with current return path 106.Sensor 120 is a hybrid design in the sense that it allows a hybrid formof measurement of conductivity by sensing the impedance across winding124. Sensor 120 provides hybrid measurement of conductivity because themeasurement is somewhat isolated from the current return path, but thetransformer-configured toroid is disposed at the sensor and maintainssome of the benefits of toroidal conductivity measurement. Thisconfiguration may provide impedance matching and/or isolation requiredto read the conductivity using direct impedance measurement techniques.In other embodiments, coupling the electrodes directly to suitabledetection circuitry may allow for direct conductivity measurement.Preferably, multiple measurement regimes (toroidal, hybrid and direct)can be selected with an electric switch (not shown).

The description with respect to the embodiments illustrated in FIGS. 4–6disclose sensors in which current flow within the process fluid isgenerated and sensed without having two or more toroids disposed aboutthe process fluid flow path. The embodiments differ somewhat in themanner in which current flow is generated and/or sensed.

FIG. 7 is a diagrammatic view of a pair of electrodes usable withembodiments of the present invention when process piping has aconductive inner surface. Process piping 200 has a conductive innersurface 202. Sensor pipe 204, which is a part of the flow-throughconductivity sensor, has a non-conductive liner 206 that extendsinwardly from each end of sensor pipe 204. Liner 206 isolates sensorpipe 204 from conductive process piping 200. A first electrode 208 isformed by a contact ring 210 disposed within pipe 204 beyond the ends212 of liner 206. Accordingly, ring 210 contacts process fluid flowingthrough pipe 204. Ring 210 is electrically coupled to conductive sensorpipe portion 214, such that electrical access to process fluid flowingthrough pipe 204 can be made through portion 214 and ring 210.

A second electrode is formed by electrically coupling to one of theconductive process pipes 200. Accordingly, a pair of electrodes 216, 218can be formed relatively easily to facilitate operation of embodimentsof the present invention with conductive process piping.

FIG. 8 is a diagrammatic view of a pair of electrodes formed withcommercially available flange pipe in accordance with embodiments of thepresent invention. FIG. 8 bears some similarities to FIG. 7 and likecomponents are numbered similarly. Electrode 216 is the same as in FIG.7. In contrast to FIG. 7, a pair of commercially available metal pipes220 are each coupled to conductive process piping 200. Each of pipes 220includes a pair of flanges 222, 224 and an insulating liner 226 thatextends from each end of the pipe 220 throughout the interior of thepipe. Conductive pipe 228 is positioned between each of pipes 220 and iselectrically isolated from process piping 200 by virtue of insulatingliners 226 on either side of pipe 228. An electrode 230 is formed usingconductive pipe 228, since electrically coupling to the outside 232 ofpipe 228 will allow electrical access to process fluid flowing throughinterior 234.

Embodiments of the present invention generally remove at least one of apair of toroids from a process fluid flow path. This allows greaterstandardization in manufacture of toroids for flow-through sensors.Disposing both toroids about a current return path provides furtheradvantages. Additionally, configuring a toroid as a transformer right atthe flow-through sensor (illustrated in FIG. 6) is believed to provide asystem that is more tolerant of the cabling used to connect the sensorto associated instrumentation.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Embodiments of the present invention areusable with non-conductive process piping as well as conductive processpiping.

1. A flow-through conductivity sensor, the sensor comprising: a flowconduit; first and second electrodes disposed relative to the flowconduit to contact process fluid proximate the conduit and convey anelectrical current between the first and second electrodes through theprocess fluid; a current return conductor coupled to the first andsecond electrodes; and at least one toroid arranged to interact with thecurrent return conductor to measure current flowing between the firstand second electrodes through the process fluid provide an indication ofprocess fluid conductance.
 2. The sensor of claim 1, wherein the atleast one toroid is disposed about the current return conductor.
 3. Thesensor of claim 2, and further comprising: a second toroid disposedabout the current return conductor; and wherein one toroid is a drivetoroid and the other toroid is a detect toroid.
 4. The sensor of claim2, wherein the at least one toroid is configured as a transformer. 5.The sensor of claim 2, wherein the at least one toroid has a pair ofwindings, and one of the pair of windings is in series with the currentreturn conductor.
 6. The sensor of claim 1, wherein at least one of thefirst and second electrodes is a contact ring.
 7. The sensor of claim 1,wherein one of the first and second electrodes includes a conductiveprocess pipe.
 8. The sensor of claim 7, wherein the other of the firstand second electrodes includes a contact ring.
 9. The sensor of claim 7,wherein the other of the first and second electrodes includes a metalpipe disposed between a pair of insulating pipes, wherein eachinsulating pipe includes insulating ends and an insulating liner.
 10. Amethod of measuring conductivity of a process fluid in a flow conduit,the method comprising: contacting the process fluid with first andsecond electrodes coupled together by a current return path; generatingan electrical current between the first and second electrodes in theprocess fluid with a drive toroid; and measuring current through thecurrent return path, the measured current through the current returnpath being indicative of the conductivity.
 11. The method of claim 10,wherein measuring includes coupling a receive toroid to the currentreturn path.
 12. The method of claim 10, wherein generating includescoupling the drive toroid to the current return path.
 13. The method ofclaim 10, wherein measuring includes directly measuring impedance of atoroid coupled to the current return path.